This invention relates to projectiles, and it particularly relates to a method of maintaining stability while reducing the aerodynamic drag on fin-stabilized projectiles and free rockets. More specifically, the projectile incorporates a low-drag, freely rotating aft section equipped with a pair of fins that provides an adequate restoring moment to the projectile during flight to provide stability in the plane in which the projectile is pitching (the pitch plane).
In the field of aerodynamics, as applied to projectile and free rockets, fins are often attached to the aft section of the projectile or free rocket to provide stability during flight. As used herein, the combination of projectiles and free rockets will be referred to by the term xe2x80x98projectilesxe2x80x99 but may be understood to refer to both projectile and free rockets. These tail fins provide a restoring moment to the projectile when there is a non-zero angle of attack, that is, when there is a non-zero angle between the projectile""s longitudinal axis and its velocity vector. The plane that contains the angle of attack is the so-called pitch plane.
In a typical configuration, 3 to 12 fixed fins are equally spaced around the circumference of the aft section of the projectile body. The location, orientation and quantity of fins ensure that sufficient lift is generated in any plane to impart the necessary moment to reduce the angle of attack to zero and, thus, stabilize the projectile.
While the multiplicity of fixed fins achieves the desired goal of providing stability to the projectile in any and all planes, it also adds undesirable aerodynamic drag, thus reducing both the velocity and range of the projectile. In particular, it can be recognized that all fins add aerodynamic drag whether or not they are producing lift necessary to minimize angle of attack.
Yet, a simple vector analysis reveals that for a conventional, fixed-fin design the maximum resulting lift is limited to a value equal to that generated by only half the fins. In contradistinction, this invention achieves stability while minimizing the aerodynamic drag on the projectile by employing a pair of fins that rotate about the longitudinal axis of the projectile to provide maximum lift in the plane in which the projectile is pitching.
Conventional, multi-finned projectiles described above have satisfied the need to provide the lift required to counteract a non-zero angle of attack and, further, to give the projectile necessary stability. However, there is still an unsatisfied need for an improved, fin-stabilized projectile that achieves overall performance via increased range and/or downrange velocities while maintaining flight-path stability.
An objective of the present invention is to provide a new aerodynamic device, such as projectile with improved flight characteristics, especially in the area of drag control. The invention achieves this objective and features by eliminating all but two of the fins required for fin-stabilized flight of projectiles.
Another feature of the present invention is to achieve enhanced overall performance of projectiles via increased range and/or increased downrange velocities as the result of low-drag flight.
Another feature of the present invention is to achieve enhanced overall performance of projectiles without adding substantial complexity to the design or implementation of the projectiles. This objective is achieved by employing a passive system for fin-stabilized flight. The passive system comprises a 2-finned tail assembly capable of rotating independently about the longitudinal axis of the main body of the projectile. With the fins free to spin about the longitudinal axis of the projectile, the existing aerodynamic forces will always orient the fins in a plane such that they provide maximum lift to decrease the angle of attack and maintain stability.
The foregoing and additional features and advantages of the present invention are realized by a projectile that includes an elongated forebody and an aft section secured to the forebody. The aft section includes a pair of fins affixed to an aerodynamic, cylindrical section. The lift generated by this low-drag pair of fins is sufficient to counteract most, if not all foreseeable angles of attack to be experienced by the projectile.
The aft section further includes a bearing that couples the aft section to the forebody of the projectile and is capable of allowing the aft section to rotate freely about the longitudinal axis of the projectile and independently of the forebody. Thus, during flight the aft section rotates into the maximum lift plane and provides a restoring moment to the projectile, thus providing necessary stability to the projectile while imparting minimum drag.